1. Field of the Invention
The present invention relates generally to a ratchet tensioner used for applying an appropriate tension to a timing belt or a timing chain of a vehicle engine, and more particularly to a ratchet tensioner equipped with a plunger releasing mechanism operative to turn a ratchet pawl in a direction to disengage a locking pawl of the ratchet pawl from rack teeth formed on a plunger.
2. Description of the Related Art
In a drive system having a timing belt or chain stretched for power transmission between an engine crankshaft and a camshaft, a ratchet tensioner is widely used for suppressing vibration generated during travel of the timing belt or chain as well as for maintaining a proper tension in the timing belt or chain. The ratchet tensioner has a spring-loaded plunger projecting outward from a tensioner housing to urge a back surface in the vicinity of the distal or free end of a tensioner lever in a direction, so that a shoe surface of the tensioner lever is held in slide contact with the slack side of a timing chain to thereby apply a tension to the timing chain. Prior examples of the ratchet tensioner are disclosed in Japanese Patent Publication No. HEI-3-8414 and Japanese Patent Laid-open Publication Nos. HEI-10-2386 and HEI-11-344086.
FIGS. 13 and 14 hereof illustrate a typical example of the conventional ratchet tensioner. This tensioner 31 includes a plunger 33 slidaby received in a plunger accommodating hole (not designated) of the housing 32 with its outer end 33A (FIG. 13) projecting outward from a front surface of the housing 32 by the force of a plunger spring 36 (FIG. 14) received inside the housing 32, and a ratchet pawl 34 pivotally mounted by a pivot shaft or pin 34A to the housing 32. The ratchet pawl 34 is accommodated in a cutout recess or groove 32E defined by two opposed side walls 32C, 32D. The ratchet pawl 34 is urged by a ratchet spring 38 to turn about the pivot pin 34A in one direction to keep meshing engagement between a locking prong 34B (FIG. 14) and one of a series of rack teeth 33B formed on an outer circumferential surface of the plunger along the axis thereof, so that the plunger 33 is prevented from moving backward.
The illustrated ratchet tensioner 31 has a built-in hydraulic shock absorbing mechanism using the flow resistance of oil. The rack teeth 33B and the locking prong 34B are designed to provide a backlash therebetween. The oil (not shown) supplied from an external oil pump operating in synchronism with an engine is introduced into a hollow interior space (not designated) of the plunger 33 via a check valve mechanism 35 provided at the bottom of the plunger accommodating hole of the housing 32. When the plunger 33 is subjected to a shock or impact force from a tensioner lever (now shown), the plunger 33 moves backward within the range of the backlash against the force of the plunger spring 36 while the prong 34B and the rack teeth 33B are kept in mesh with each other. In his instance, the check valve mechanism 35 is closed so that the oil within the hollow interior space of the plunger 33 is forced to leak out from a clearance between the outer circumferential surface of the plunger 33 and an inner peripheral surface of the plunger accommodating hole of the housing 32 to thereby absorb the impact force. Reference character 32B shown in FIGS. 13 and 14 denotes holes used for mounting the ratchet tensioner 31 to the engine body.
The ratchet tensioner 31 further has a lever 37 (FIG. 13) firmly connected at one end to an end of the pivot pin 34A and projecting outward from the housing 32. By turning the lever 37, the ratchet pawl 34 is turned about the pivot pin 34A in the direction of arrows shown in FIGS. 13 and 14 with the result the engagement between the ratchet prong 34B and the rack teeth 33B is released.
When the engine is to be repaired or when the chain is to be removed, the plunger 33 of the ratchet tensioner needs to be displaced in a fully retracted position. In the conventional ratchet tensioner 31, the plunger 33, which is operating as a one-way mechanism, can be released by turning the ratchet pawl 34 in the direction of arrows against the force of the ratchet spring 38. However, the conventional ratchet tensioner 31 has no means or mechanism for maintaining the plunger 33 in the released state. It is, therefore, difficult to keep the locking prong 34B of the ratchet pawl 34 separated from the rack teeth 33B of the plunger 33. This makes it difficult to achieve the chain replacement operation with ease. In addition, the lever 37 cannot be easily turned or manipulated by a screwdriver which is inserted in a hole 37A in the lever 37 through a small service hole provided in the engine. Even when the lever 37 is turned by using such screwdriver, a difficulty still arise in maintaining the lever in an angularly displaced condition. As a consequence, it is practically impossible for the plunger 33 to be maintained in a released condition with the locking prong 34B disengaged from the rack teeth 33B. Another problem is that the lever 37 attached as a separate part to the pivot shaft 34A of the ratchet pawl 34 increases the number of part of the tensioner 31, which will incur an additional cost.
Accordingly, a general object of the present invention is to solve the aforementioned problems associated with the conventional ratchet tensioners.
A more specific object of the present invention is to provide a ratchet tensioner having a plunger releasing mechanism which can readily turn a ratchet pawl against the force of a ratchet spring in a direction to disengage prongs of ratchet pawl from rack teeth on a plunger, thereby keeping the plunger in a released condition.
To achieve the objects, according to the present invention, there is provided a ratchet tensioner comprising: a housing; a plunger reciprocally mounted in the housing and urged by a plunger spring received inside the housing so that one end of the plunger projects outward from the housing; a ratchet pawl pivotally mounted by a pivot shaft to the housing within a recessed groove formed in the housing, the ratchet pawl having a locking prong at one end thereof located on one side of the pivot shaft; and a ratchet spring urging the ratchet pawl to turn about the axis of the pivot shaft in one direction so that the locking prong of the ratchet pawl is held in mesh with one of a series of rack teeth 3B formed on an outer circumferential surface of the plunger along the axis thereof. The ratchet tensioner further includes a plunger releasing mechanism for turning the ratchet pawl about the axis of the ratchet shaft in the opposite direction against the force of the plunger spring to disengage the locking prong of the ratchet pawl from the rack tooth of the plunger. The plunger releasing mechanism has a pin insertion hole formed in the housing transversely across the recessed groove for permitting insertion of a pin from the exterior of the housing, the pin insertion hole being located on the opposite side from the locking prong with respect to the pivot shaft, and a sloped surface formed on the ratchet pawl and normally extending obliquely across the axis of the pin insertion hole. The sloped surface, upon abutment with a tip end of the pin which is inserted in the pin insertion hole, is capable of causing the ratchet pawl to turn in the opposite direction against the force of the ratchet spring to thereby disengage the locking prong of the ratchet pawl from the rack tooth of the plunger for releasing the plunger.
In one preferred form of the present invention, the ratchet pawl has a pin accommodating hole formed therein with the axis normally offset from the axis of the pin insertion hole of the housing. The pin accommodating hole has an inner circumferential surface portion extending at an angle to the axis of the pin accommodating hole and forming the sloped surface.
The pin insertion hole is open at least at one end thereof. The pin accommodating hole may comprise a tapered hole having a larger end facing in the same direction as the open end of the pin insertion hole and a smaller end opposite to the larger end. The larger end of the tapered hole has an oblong shape, and the smaller end of the tapered hole has a circular shape, the oblong-shaped larger end of the tapered hole and the circular-shaped smaller end of the tapered hole being connected by the sloped surface. As an alternative, the larger end of the tapered hole has an oblong shape and an inner circumferential surface extending parallel to the axis of the tapered hole, and the smaller end of the tapered hole has a circular shape and an inner circumferential surface extending parallel to the axis of the tapered hole. The sloped surface is formed by a portion of an inner circumferential surface of an intermediate portion disposed between the oblong-shaped larger end of the tapered hole and the circular-shaped smaller end of the tapered hole. The pin accommodating hole may be a circular hole with countersink having a larger end facing in the same direction as the open end of the pin insertion hole and a smaller end opposite to the larger end. The countersink has an inner circumferential surface partly forming the sloped surface.
In another preferred form of the present invention, the ratchet pawl is beveled at an edge so as to form the sloped surface.
When the pin is inserted in the pin insertion hole, a tip end of the pin engages the sloped surface of the racket pawl. As the insertion pin further advances, the tip end of the pin forces the sloped surfaces in the forward direction. By virtue of a horizontal component of the pressure or force applied from the pin to the sloped surface, the ratchet pawl is turned about the pivot shaft in the opposite direction against the force of the ratchet pawl to disengage the locking prong of the ratchet pawl from the rack tooth on the plunger. The plunger is thus released. When the pin is fully inserted in the pin insertion hole across the recessed groove, the ratchet pawl is held in a releasing position in which the locking prong is disengaged from the rack tooth of the plunger and thus allows the plunger project by the force of the plunger spring or to be manually displaced backward in a fully retracted position.